A charged body or charge producing body moving in an electrostatic field interacts with or disturbs that field. The interaction or disturbance can be detected at any point in the electrostatic field based on the principle's of Coulomb's law. These field effects can be readily modeled by derivation of equations based on Coulomb's law, as is well known to those skilled in the art. Thus, for example, a charged body or a body producing a charge, such as an aircraft moving through such a field can be detected by electrostatic detection means, such as an electrostatic gradiometer, located on another aircraft moving through the field relative to the first aircraft, or such a device located on the Earth.
Single and paired electrostatic field probes for measuring the magnitude of an external electrostatic field are well known in the art. Likewise, probe configurations and combinations to compensate for deleterious environmental and circuit intrinsic effects are known. A typical application of paired probes to an aircraft electrostatic field problem relates to determining the potential between an aircraft and electrical ground to ascertain the charge buildup on the aircraft skin that must be safely discharged. Further, skin charge buildup creates radio frequency interference. Thus, the use of an aircraft skin discharge system to bleed skin charge and effectively minimize its buildup is a necessity to protect personnel from high voltage shock and to reduce radio frequency interference to communications and to onboard avionics equipment. Skin charge can create a voltage differential between an aircraft and the ambient environment as high as 160 kilovolts unless reduced in some measure by means of charge bleed-off The disclosure of U.S. Pat. No. 4,005,357 is incorporated by reference herein
The use of one or more probes to determine the presence, location, and gross or specific identity of a charge bearing or charge generating vehicle such as an aircraft moving into the electrostatic field in which said probes are located does not appear to be known in the art.
An aircraft is an example of a particular type of charged body or charge producing body which, when moving in an electrostatic field, will disturb that field in a manner which will permit its detection by means of a field gradiometer system such as the subject invention. Whether fixed wing or helicopter, a moving aircraft picks up a measurable amount of body or skin charge. The skin charge is typically negative. In addition to skin charge, a powered aircraft typically produces engine exhaust stream ions. Exhaust stream charge occurs generally as a function of throttle position. Thus, when the throttle is positioned for acceleration, the engine exhaust contains charged particles giving it a net positive charge. The flow of these charged particles in the exhaust stream can presently be measured with existing devices as currents ranging from 50 to 400 microamperes. The positive exhaust stream charge is generally less in absolute magnitude than the negative skin charge.
It should be recognized that both environmental and aircraft operating conditions will affect the amount of charge created on the aircraft skin or in the engine exhaust. Thus, an aircraft skin discharge system may bleed skin charge to effectively minimize its buildup. Throttle control, on the other hand, is one means of minimizing exhaust stream charge buildup. However, the need for exhaust stream charge and skin charge management for the purpose of detection avoidance has not been an issue in the prior art. The present invention is capable of detecting aircraft whether charge management is employed or not.
In addition to determining the presence of a charged body, such as an aircraft, moving in an electrostatic field, and determining its operating characteristics, experiments have shown that particular types of aircraft, and particular models create recognizable signatures based on skin charge and exhaust plume charge, as a result of their interaction with an electrostatic field. Obviously, other types of vehicles which move and which have ion producing engine exhausts, such as tanks, will similarly create electrostatic field disturbances or interactions peculiar to such vehicles and which can be detected and recognized with the subject invention.
In the present invention the electrostatic field detectors are employed in pairs with one detector physically spaced a known distance from the other, optimally determined for a particular application. The detectors can also be employed in arrays or other ordered arrangements, including orthogonal arrays, or for some applications they may be seeded, randomly or otherwise in a manner constituting unordered array placement. Where more than a two detector system is involved, a means is required for processing the signals in the multi-detector arrangement or unordered placement, to determine the optimum signals which in combination produce an output representing a signature containing characteristics of the charged body moving in and disturbing the field. Such signal processing technology, presently evolving in the state of the art, is directly applicable to this concept.
The signals produced by each probe may be ion enhanced by placing a source of ionization on the probe. A radioactive material producing alpha particles, such as, for example, polonium, is used in the present invention to produce a charge or ion corona about the probe. When the field around the probe is then disturbed, charge flow within the probe is supplemented with charged particles from the corona, and thus increased. In general, however, probe signals are low level. Signal levels in the voltage mode of operation, versus the alternative current mode of signal measurement, are typically in the order of one milli volt per meter using a commercially available wide band measurement system. This system, using a 30 meter probe separation demonstrated target detection at approximately one kilometer. Calculations indicate that modern signal processing and devices should be capable of a signal detection range, for a typical aircraft charge, in excess of 30 nautical miles for a 10 meter probe seperation distance.
Whether the probes are mounted on an aircraft for detection of another aircraft or located on the ground, signal transmission cables connecting the probes to the signal processing elements of the invention, such as the high impedance amplifiers and differential amplifiers, or combinations of these amplifiers in an electrostatic voltmeter configuration must be kept short and selected for low loss transmission characteristics. Thus, cables having higher characteristic impedance, such as 75 ohm versus 50 ohm coaxial cable, and having low capacitance per unit length, provide better voltage signal transmission over greater distances. For particular applications such as remote relays, fiber optic and radio frequency links provide the necessary low loss signal transmission capability. In addition, for ground based probe arrays or seeded placements where the probes are spaced apart more than 10 meters, signal pre-conditioning, such as low noise pre-amplification or conversion to optical signals prior to transmission for further processing are employed. From the foregoing, and with an appreciation of the low magnitude of the signals produced by the electrostatic probes in the sensory portion of the invention, it should be understood that the signal processing function of the invention appearing between the output of said probes and the point at which signal information content is made usable by being presented on a display, is related to the application.